Radiation therapy delivered to the whole breast is standard treatment following tumor removal for patients with early stage breast cancer. In selected patients, delivering the radiation treatment to only the part of the breast involved by the tumor may be a viable alternative. This can decrease the amount of normal tissues exposed to irradiation, potentially decrease side-effects and shorten the treatment time. Proton beam can decrease the volume of normal tissues treated compared to standard forms of radiotherapy. This study will recruit patients with early stage breast cancer following surgical removal to undergo partial breast radiotherapy with proton beam and determine the recurrence rates and side-effects of treatment.